Wearable Devices Made of a Wireless Vertical-Type Light-Emitting Diode Package on a Flexible Polyimide Substrate with a Conductive Layer

In this study, wearable devices are made using wireless vertical-type light-emitting diode (LED) packages of a transparent conductive film coated on flexible colorless polyimide (PI) with 50 μm thickness. The low-stress ultrathin transparent conductive multilayers are deposited on the PI using high-power impulse magnetron sputtering at 65 °C. It can be used as the electrode in an ultraflexible photoplethysmography (PPG) biosensor. The nearly stress-free multilayer, consisting of a Ag layer with 20 nm thickness sandwiched between indium tin oxide (ITO) layers with 30 nm thickness, is transparent to infrared (940 nm) and exhibits a sheet resistance and resistivity of 5.7 Ω/sq. and 4.57 × 10–5 Ω·cm, respectively. We fabricate a PPG biosensor with a vertical-type infrared LED bonded onto the flexible ITO/Ag/ITO/PI. We demonstrate that our PPG biosensor is a sensitive and accurate screening tool for the detection of pulsation signals and the heart rate (HR). The standard deviation of HR detections is as low as 2.6 bpm, satisfying the standard of <5% for approximately 3.5 bpm set by the AAMI SP10/ISO 81060-2:2009. This kind of a PPG biosensor can be applied to wearable devices for healthcare monitoring applications.